1. Field of the Invention
The present invention relates to a method and apparatus for the transmission of information by radio using a middle channel, a first side channel and a second side channel in a radio system.
2. Description of Related Art
The term electronic location and tracking refers to systems which can be used to locate and, if necessary, to follow a desired object as it moves with predetermined degree of accuracy and within a desired geographic field or area or extent by using radio transmission and receiving methods. One application in which such location service is a desired property is the national safety service network called SafeNet. The principle functions of SafeNet are the transfer and routing of supervisory information, the adaptation of different safety systems and networks for integrated utilization, and the establishment of a simple user interface to provide access to different telecommunication services needed in the safety sector. SafeNet has been designed to provide standard interfaces for the most common object-under-surveillance systems and to combine, in the same service network, the telecommunication services needed by the operative functional units. The radio communication must be encrypted and must be able to provide a sufficient data transmission capacity for SafeNet purposes and properties.
Achieving of the above-described attributes and functionality is the principal object of the present invention. It is a particular object of the invention to provide a novel methods and apparatus for the transmission of information by radio.
In accordance with the invention, information is transmitted using a single middle channel, a first side channel and a second side channel in a radio system that includes a channel transmitter unit and a channel receiver unit which have been configured to function at a plurality of radio frequencies. In the inventive method, the width of the middle channel frequency band is defined and the widths of the side channel frequency bands are so defined that they are equal to or smaller than the width of the middle channel frequency band. The middle channel is transmitted as a frequency-modulated (FM) radio signal that is created by modulating the middle channel by a suitable frequency. A signal that is symmetrically amplitude-modulated (AM) with respect to the center or mid-frequency of the middle channel is also generated; in other words, the frequency of the carrier wave of the amplitude-modulated signal is substantially the same as the mid-frequency of the middle channel.
The middle wave and the upper single side band of the amplitude-modulated signal are suppressed by taking a sample of the middle wave and the upper single side band, reversing the phase of the sample and summing it with the original amplitude-modulated signal to thereby produce a first amplitude-modulated single side band. A second amplitude-modulated single side band is similarly formed from the amplitude-modulated signal by suppressing the middle wave and the lower single side band by taking a sample of the middle wave and the lower single side band, reversing the phase of the sample and summing it with the original amplitude-modulated signal.
The first single side band is modulated by the information in the first side channel, thus forming two side bands for the first single side band. The second single side band is modulated by the information in the second side channel, thus forming two side bands for the second single side band. The frequency-modulated signal, the first single side band and the second single side band are then combined in a combiner for simultaneous transmission of all three channels.
In a preferred embodiment of the inventive method, the first and second single side bands are transmitted substantially in phase with each other. The middle channel is transmitted in substantially opposite phase relation to the phase of the first and second single side bands.
In accordance with another preferred aspect of the inventive method, an analog signal is transmitted in the middle channel and a digital signal is transmitted in at least one of the side bands.
In another preferred aspect of the inventive method, the carrier frequency used for reception is either the frequency-modulated signal, the first single side band or the second single side band.
In still another preferred aspect of the inventive method, an encryption key code for the middle channel is transmitted in one of the single side bands, the key code allowing an encrypted message to be decrypted.
The inventive apparatus further provides a transmitter unit for the transmission of a middle channel, a first single side band and a second single side band in a radio system. The radio system comprises a channel transmitter unit and a channel receiver unit which have been constructed or configured to function at a plurality of suitable radio frequencies. The transmitter unit includes means for generating a suitable transmission frequency and means for modulating the middle channel into a frequency-modulated (FM) form. The transmitter unit also includes means for generating an amplitude-modulated (AM) signal such that the carrier wave is at the center or mid-frequency of the middle channel. Thus, the carrier waves of the amplitude-modulated and frequency-modulated signals are at substantially the same frequency. The transmitter unit additionally includes means for forming a first and a second single side band from the amplitude-modulated signal, means for modulating the first single side band by the first side channel, means for modulating the second single side band by the second side channel, and means for combining the first single side band, the second single side band and the middle channel for simultaneous transmission.
In a preferred embodiment of the inventive apparatus, the means for forming the first and second single side bands comprises means by which a carrier wave to be suppressed and one of the upper and lower single side bands of the amplitude-modulated signal are substantially reversed in phase and then summed with the amplitude-modulated signal. The transmitter unit preferably includes means for phasing the first and second single side bands so as to make them substantially cophasal. The transmitter unit also preferable includes means for phasing the middle channel so as to make it substantially opposite in phase to the phase of the single side bands.
The present invention advantageously provides for the combination and transmission of three information channels in a single signal, thereby obviating interference between signals. The invention may be utilized to implement an encryption function, as for example one that is appropriate for or required in SafeNet applications, with the side channels being modulated by encryption keys. The inventive apparatus is readily implemented using existing materials and components, and the efforts and procedures required to implement the inventive method are relatively straightforward and simple.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawing. It is to be understood, however, that the drawing is designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.